Yeah, i know about that rule too, i learned it at school, just with "proud" both options sounded good to me, and that made me doubt. That and the fact that "prouder" is not at the wordreference dicctionary.

Thank you both for your answers, but i still don't really know which one to use lol since each of you chose a different option hahahaha.

I guess that when you want to reinforce the idea, then you say 'more proud'. Consequently, depending on each situation of communication, you would use it, just as paolorausch pointed out above.

One example I can think of is the sentence: "nothing would make me more proud". Saying "nothing would make me prouder" is, of course, correct. However, saying one or the other depends on your intention. Wanna emphasize the message? Use the first example. You don't find necessary? Opt for the second one.

Some adjectives appear less frequently in the comparative form, and therefore one hesitates to be dogmatic. In the example just given, I would probably say Nothing would make me prouder, but I would not rule out the other form.